Power semiconductor modules typically include a base plate which is pre-bent to have a convex shape after one or more substrates are joined to the base plate. The convex shape provides better thermal impedance after mounting the opposite side of the base plate to a heat sink. A thermal interface material is provided between the substrate and base plate, and becomes a relatively thin layer in case of a convex shape of the base plate. During mounting with screws which are placed along edges of the base plate, the base plate is bent downward at the edges to be more or less flat with the heat sink. The bending of the base plate results in a bending of the substrate and semiconductor die mounted on the substrate.
Depending on the kind of joining process and conditions employed, the joining layer between base plate and substrate may be coplanar to the base plate, convex or concave shaped. As such, the substrate does not necessarily have a flat shape after joining to the base plate. In any case the bending of the base plate and related bending of the substrate under a thermal load may cause cracks to propagate in the ceramic substrate. Another stress results from the thermal load. Due to a thermal expansion mismatch between the substrate and base plate, the entire module is predisposed to become more convex under thermal loading. The base plate cannot bend under a thermal load because the base plate is fixedly clamped flat on the heat sink. Thus the stress within the substrate (particularly in the ceramic material of the substrate) rises even further during heating, causing additional cracking and damage.
Another problem arises due to the thermal expansion mismatch. The thermal interface material provided between the base plate and substrate tends to be paste-like. As the deformation of the entire module system during heating is complex, heating and cooling may cause the thermal interface material to be pumped out from between the substrate and base plate. The thermal resistance of the interface between the base plate and substrate increases as more thermal interface material is pumped out, degrading the overall thermal performance of the module.